Antiferromagnetic order in MnO spherical nanoparticles

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8- and 13-nm antiferromagnetic MnO nanoparticles. For the 8-nm sample, the antiferromagnetic transition temperature T_N (114 K) is suppressed compared to that in the bulk material (119 K), while for the 13-nm sample T_N (120 K) is comparable to that in the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2±0.3 μ_B/Mn for the 8-nm sample and 3.9±0.2 μ_B/Mn for the 13-nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80% of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.